Use scrollToPositionWithOffset in widget list and account for padding

Also, scroll after collapsing as well to preserve the header position.
LinearLayoutManager seems to not count/double count the top padding of
the RecyclerView when scrolling to the position. Scrolling with the
offset and deducting the top padding works around the issue.

There's still some occasional weirdness that needs to be investigated,
but this works well...most of the time.

Bug: 183378651
Test: locally
Change-Id: I0ba85fb65411991ef781f08a69faaa993a7d7fd0